Controllers for current disk drives often include a disk formatter circuit that interfaces to read channel circuitry via a non-return to zero (NRZ) bus. More specifically, the disk formatter circuitry performs both data path and control path operations, and typically receives timing information and data so as to control a sequence of read or write operations. The read channel circuitry is responsible for actual reading of data from the physical disk medium, and the writing of data thereto. The read channel circuitry and the disk formatter circuit interface with each other via the NRZ bus, which often has a bit depth of 10 bits.
With increasing performance of the hard disk drive, both in terms of speed of data transfer to and from a host as well as storage capacity and density, there is a corresponding increase on the performance demands of the NRZ bus and the disk formatter circuit. Testing is performed to confirm proper operation. Testing may be performed at manufacture time or at any time before or during deployment of the hard disk drive in actual service. However, these increases in performance also make it more difficult to perform testing accurately, since the test equipment itself might not have performance capabilities that are sufficient to detect and report on disk subsystem failures, or to confirm that these subsystems are performing correctly.